1. Technical Field of the Invention
The invention relates to the automatic controls enabling certain systems to be maintained about a particular operating point. In a preferred application, the invention concerns methods for testing tires. In another preferred application, the invention concerns systems for controlling the stability of a vehicle, more particularly in their function aimed at avoiding locking of the wheels when the brakes are applied, popularly known by the term “ABS”, but also in their more sophisticated versions aimed at maintaining the vehicle on a stable path by automatically acting on the brakes of one wheel, such as for example in the systems popularly known by the term “ESP”, or by acting on any other actuator (four wheels steering, active anti-roll, . . . ).
2. The Related Art
It is known that the braking of a vehicle will be all the more efficient if the tire tread is made to function at a slip G corresponding to the maximum value of the coefficient of friction. The maximum value of the coefficient of friction is called μmax. But the average driver is not capable of controlling the braking so as to meet this condition.
The first so-called “ABS” brake systems automatically modulated the brake force (actually, the braking actuator generally being now a hydraulic jack, an ABS system modulates the hydraulic pressure) so as to cause the functioning of the tire to oscillate about the maximum grip. This involves exceeding the maximum grip in order to be able to detect it by initiating the locking of the wheel (sudden deceleration of the rotation of the wheel), before reducing the brake force so as to be just below the maximum grip again. The brake force is then automatically increased again until it exceeds the maximum grip, then reduced, and so on.
Nevertheless, this method involves briefly exceeding the slip Gmax corresponding to the maximum value of the coefficient of friction μmax, whereas the ideal situation would be to approach the target slip by default without ever exceeding it. It is important to note that one calls Gmax, conventionally, not a maximum possible value of the slip, but actually the particular slip at which the friction coefficient has its maximum possible value.
The efficiency of the braking depends on the fineness of the slip variations about the slip corresponding to the maximum coefficient of friction. When efficiency is referred to, the only concern here is the amount of the deceleration, putting aside the major benefit of ABS systems of affording the driver of the vehicle a certain capability to cause the latter to turn during emergency braking. Consequently, in the context of the present invention, braking is considered as being all the more efficient, the shorter the braking distance. The efficiency of such braking is impaired by the periods in which the braking is not at the level of the coefficient of maximum grip, that is to say, during periods of excessive slip and during periods of insufficient slip.
The first so-called “ABS” brake systems, the functioning of which has been mentioned above, had the advantage of automatically adapting to the various tires. This feature is important since it is known for example that the slip of a winter tire at the maximum coefficient of friction is considerably greater than the slip of a summer tire at the maximum coefficient of friction, just as it is known that the slip of a new tire at the maximum coefficient of friction is greater than the slip of a worn tire at the maximum coefficient of friction. Unfortunately, the vibrations caused by this type of automatic control are unpleasant and may even have the effect that the driver releases the pressure on the brake pedal. This generation of brake system is illustrated for example by U.S. Pat. No. 3,980,346, in which an improvement of such a system is described.
This system enables adaptation to various tires. To do this, the pressure is increased in stages. The development of the rotational speed of the wheel is then observed, from which it is then deduced if the pressure needs to be increased or decreased. Such automatic control is “adaptive” but naturally generates vibrations.
At present, vehicle stability control systems automatically modulate the brake force so as to aim at a predetermined target slip, which is supposed to correspond to the maximum coefficient of friction.
In this case, a vehicle brake system therefore aims to maintain a brake force such that the tread functions at the optimum level of slip chosen. Such a system continuously measures the rotational speed of each of the wheels VTire. With a specific algorithm (see for example U.S. Pat. No. 5,402,345), an estimation of the vehicle speed VVehicle is obtained. An estimation of the instantaneous slip G=1−VTire/VVehicle is therefore available. Ideally, as long as this estimated slip remains below the optimum slip, the brake force does not have to be lowered, or may even be automatically increased if a function for automatic brake boosting is activated (see for example U.S. Pat. No. 5,816,666). When the greatest possible brake force is attained, the brake pressure is regulated so as to maintain an optimum slip Gmax, that is to say, the slip corresponding to the maximum coefficient of friction (μmax).
All that remains is to determine the optimum slip. In EP patent application 0503025, this is done from a reference curve giving a value of G to be aimed for as a function of the estimated coefficient of friction μ and the likewise estimated vehicle speed. An estimation of the coefficient of friction μ is carried out as follows. When braking in a straight line on a homogeneous ground, the brake force FX of the tire on the ground is determined from the brake pressure and the construction parameters of the wheel and of its brake. With the knowledge of all the forces FX applied by all the tires, it is possible to calculate the deceleration of the vehicle, and, therefore, taking account of the vehicle characteristics, the load transfer, and therefore the load variations on each of the wheels. From this, it is possible to deduce an approximation of the vertical load FZ applied to each tire. An estimation of the coefficient of friction μ=FX/FZ is thus obtained. If the corresponding lateral force Fy is known, by estimation or measurement, a more precise estimation of the coefficient of friction is given by the formula   μ  =                                          F            X            2                    +                      F            Y            2                                      F        Z              .  In the context of the present invention, these two estimations will be considered as equivalent. Similarly, and this is obvious to a person skilled in the art, in the context of the present invention, everything that has been stated about braking is valid in the case of acceleration; in other words, a braking force is, as regards the considerations relating to grip, equivalent to a driving force, even if, of course, the actuators for modifying these are not the same.
Furthermore, by referring to the aforementioned reference curve, it is established what the reference coefficient μ for the estimated slip G would be. As long as the current estimated slip is below the target slip, the slip is increased until the slip values substantially coincide. An advantage of this second system is that there are fewer oscillations about the maximum slip than with the first.
Unfortunately, this reference curve is predetermined experimentally, and therefore for a limited number of tires, and is unable to take account of the actual state of the vehicle tire equipment, beyond these conditions of use, for example inflation pressure, level of wear, etc. Although this automatic control principle actually enables the vibrations to be limited or eliminated, the braking efficiency is all the more impaired since the tire actually used intrinsically requires a slip at the maximum coefficient of friction which is very different from that programmed in fact in the reference curve.